Theodor Seuss Geisel, who wrote the Dr. Seuss books, put the manuscript and illustrations aside more than 50 years ago. The author’s widow, Audrey, found the long-lost pages after his death in 1991—then misplaced them—and found them again in 2013.

“I think he set it aside, mostly because he’d gotten involved in other things,” Cathy Goldsmith, a collaborator of Dr. Seuss’ told PBS News. “I think this was one he wrote, and moved on almost immediately to ‘One Fish, Two Fish.’”

Goldsmith worked as a designer and art director with Dr. Suess on his last six books. She helped choose the color style of this latest discovery based on other books he’d written around same time in 1960.

“What Pet Should I Get?” is about learning to make difficult decisions. In the book, two children face the task of choosing just one pet, but like all the animals they see and time is running out:

“We have to pick ONE pet and pick it out soon. You know Mother told us to be back by noon.”

Even in an age of tablets and e-readers, Random House Children’s Books never doubted the new book would be popular. They had originally planned for 500,000 copies in their first printing, but expanded it to one million after it became an instant best seller when it went on sale Tuesday.